Frank 'Gud' Coleman, a 43-year-old Ngemba man, was found on Thursday in his cell at Long Bay jail.

Coleman, who was born in Brewarrina in northwest NSW and raised in Mount Druitt in western Sydney, is the ninth Indigenous person to die in an Australian prison since March.

At least 478 Indigenous people have died in custody since the 1991 Royal Commission into Aboriginal Deaths in Custody.

Coleman's family and the Aboriginal Legal Service are demanding answers after learning he was found unresponsive.

His former partner Skye Hipwell, who is the mother of three of his children, said he was healthy and his death had come as a "complete shock".

The family's grief is compounded by the fact they hadn't seen him for more than a year due to COVID-19 restrictions in NSW jails.

"He died alone in a jail cell and no one knows anything about his last minutes," Ms Hipwell said.

She described him as an outgoing, friendly man who loved his children and was "extremely staunch and proud in his Aboriginality, his culture and community".

His daughter Lakota Coleman said he had a heart problem but was taking medication so the death came as a shock.

"We were told from the nurses from Long Bay jail that he was in good health," she told ABC TV on Wednesday.

Coleman's family want an autopsy and coronial inquest as soon as possible.

A Corrective Services spokeswoman said Coleman was found unresponsive in his cell at approximately 5.45am before he was pronounced dead at 6.26am.

All deaths in custody were subject to a coronial inquest and Corrective Services NSW and NSW Police are investigating the incident, she said.

Oxfam Australia Chief Executive Lyn Morgain said it was a "sad and shameful day" for Australia.

"While the circumstances that led to Frank's death will be the subject of an inquiry, there can be no disputing the fact that far too many First Nations women, children and men are incarcerated in Australia, many of them for minor offences," Ms Morgain said.

"There has been a woeful and unacceptable lack of progress on tackling Indigenous incarceration."

Karly Warner, chief executive of the Aboriginal Legal Service for the NSW and ACT, said it was devastating to be dealing with "yet another" Indigenous family whose loved one died alone behind bars.

"Life-saving recommendations (from the 1991 commission) remain on the shelf while people like Frank suffer lonely and preventable deaths," Ms Warner said.

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Australian Associated Press